404 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



have gills instead of lung sacs and water breathing in- 

 stead of air breathing. In Fig. 283 we show in diagram 

 the relation of heart and gills in these. 



In many lower forms {Nudibranchiata) the gills con- 

 sist of external tufted projections from the skin, waving 

 in the water. 



Cephalopoda. — These are the highest of mollusca. 

 They are always water breathers, but their gills are 

 quite complex and efficient. The gills of these animals 

 lie on each side, in the dorsal region, and the breathing 

 is by the contraction of the hol- 

 low water-filled muscular man- 

 tle, which throws out the water 

 throughthe siphon (see Fig. 226, 

 page 341), and its own elasticity 

 restores again its form and draws 



FIG 284 -Diagram of heart j n the fregh water 

 and gills of dibranch Ceph- 



alopod: v, ventricle ; au, Circulation. — In dibranchs 



auricle ; br, branchiae. . 



(two-guled) or naked cephalo- 

 pods, such as the squid and cuttlefish, there are two auri- 

 cles, one to each gill ; in tetrabranchs (four-gilled) or 

 shelled cephalopods, such as the nautilus, there are four 

 auricles. In all there is but one ventricle. The relation 

 of the heart to the gills is seen in Fig. 284. 



ECHINODERMS. 



As an example of these we take the starfish (As- 

 ter ias). 



Respiration. — The respiration of these is performed 

 in two ways : 1. The whole body is hollow and the body 

 cavity is only partially filled with the viscera, leaving a 

 large perivisceral space filled with water. Contraction 

 of the whole body presses out the water through a mul- 

 titude of pores (Fig. 285, a a), while fresh water is drawn 

 111 by the restoration of the body form. The contained 



